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In my last article on brain size and IQ, I showed how people with half of their brains removed and people with microcephaly can have IQs in the normal/above average range. There is a pretty large amount of data out there on microcephalics and normal intelligence—even a family showing normal intelligence in two generations despite having dominantly inherited microcephaly.

The cerebral cortex makes up only 86.4% of brain volume when measured by MRI (Filipek, Richelme, Kennedy & Caviness, 1994), so the total brain volume of the 10-year-old would be larger at 850.7 cc. Brains at 10 years are about 4.4% smaller than adult size (Dekaban & Sadowsky, 1978), suggesting that that brain would grow to an adult size of 888 cc. Even using the lower figure of 80% cerebrum to brain ratio derived from anatomical studies suggests a figure of only 960 cc.

The variation of 888 cc to 960 cc depending on which value for the cerebrum to brain ratio you use still shows that people can have brains 450-300 cc lower than average and still be ‘normal’.

If we can have smaller heads—which would make childbirth easier and allow us to continue to have smaller pelves which would be conducive to endurance running since we are the running ape, why would brains have gotten so much larger from that of erectus (where modern people can have normal IQs with erectus-sized brains) if it is perfectly possible to have a brain on around the size of early erectus? In any event, these anomalies need an explanation, and Skoyles (1999) hypothesizes that people with smaller heads but normal IQs may have a lower capacity for expertise. This is something that I will look into in the future, as it may explain these anomalies, along with the true reason why our brains began increasing around 3 mya.

Sells (1977)—using the criteria of 2 SD below mean head size—showed that 1.9 percent of the children he tested (n=1009) had IQs indistinguishable from their normocephalic peers. Watemberg et al (2002) studied 1,393 patients. They found that almost half of their patients with microcephaly (15.4% of their patients studies had microcephaly) had IQs within the normal limits, while among those with sub-normal intelligence, 30 percent had borderline IQs or were mildly mentally retarded (it’s worth noting that l-glutamate can raise IQ scores by 5-20 points in the mild to moderate mental deficiency; Vogel, Braverman and Draguns, 1966 review numerous lines of evidence that glutamate raises IQ in mentally deficient individuals). Sassaman and Zartler (1982) showed that 31.9 percent of microcephalics had normal intelligence, 6.9 percent of them had average intelligence.

Head circumference does not directly correlate with IQ in microcephalic patients (Baxter et al, 2009). Dorman (1991: 268) writes: “Decreased head size may or may not be associated with lowered intelligence, indicating that small head size by itself does not affect intelligence. The presence of subgroups of microcephalic persons who typically have normal intelligence is sufficient to rule out a causal relationship between head size and intellect. … It can be added that reduction in brain size without such structural pathology, as mayvoccur in some genetic conditions or evenvas a result of normal variation, does not
affect intelligence. ”

Tenconi et al (1981) write: “We were able to examine five other members of this family (1-3; 11-1; 11-4; 11-5; 11-8) and found no abnormalities: they were of normal intelligence, head circumference, and ophthalmic evaluation. Members of the grandmother’s family who refused to be examined appeared to be of normal intelligence and head appearance and did not have any serious eye problems.”

Stoler-Poria et al (2010) write: “There was a K-ABC cognitive score < 85 (signifying developmental delay) in two (10%) children from the study group and in one (5%) child from the control group: one of the children in the study group (the one with HC below − 3 SD) scored significantly below the normal range (IQ = 70), while the other scored in the borderline range (IQ = 83); the child from the control group also scored in the borderline range (IQ = 84).” Whereas Thelander and Pryor (1968) showed that individuals with head circumferences 2-2.6 SDs below the mean had average IQs, though the smaller their HC, the lower their IQ. Ashwal et al (2009: 891) write: “The students with microcephaly had a similar mean IQ to the normocephalic group (99.5 vs 105) but had lower mean academic achievement scores (49 vs 70).” So it seems that microcephalics can have normal IQs, but have lower academic achievement scores.

There is one case study of a girl with microcephaly where Tan et al (2014) write: “Most recent measures of general intelligence (performed at 6½ years of age) reveal a below average full scale IQ of 75 with greatest impairment in processing speed. On the Wechsler Preschool and Primary Scale of Intelligence III Revised (for children 2 years 6 m – 7 years 3 m), she obtained a Verbal IQ of 83, Nonverbal IQ of 75, and Processing speed 71. On the Wechsler Individual Achievement Testing (WIAT) she showed significant struggles in secondary language on tasks of early reading (SS 60), word reading (SS 70), reading comprehension (SS 69) and struggles in math on the task of numerical operations (SS 61) (WPPSI – R and WIAT mean = 100 and SD = 15). Parents report subjectively that differences in development relative to her sisters are becoming more apparent with time.”

In conclusion, microcephalics can have normal IQs and live normal lives, despite having heads, on average, that are 2 SDs below the mean. These anomalies (and there are many, many more) need explaining. This is great evidence that a larger brain does not always mean a higher IQ, as well as yet more evidence that it was possible for Homo erectus to have an IQ in our range today, which means that we may not need brains our current size for our intellect and achievements. To conclude, I will provide a quote from Dorman (1991):

The normal intelligence found by SELLS in school children with small head size also militates against any straightforward relationship between diminished head size and lowered intelligence.

With the correlation between brain size and IQ being .4 (Gignac and Bates, 2017), this does not rule out the ‘outliers’ reviewed in this article. These cases deserve an explanation, for if large brains lead to high IQs, why do these people with heads significantly smaller have IQs in the normal range? (See Skoyles, 1999: 8, para 31 for an explanation for the brain size/IQ correlation.)

21 Comments

Very good article! Something that hereditarians have an hard time to explain.

It also seems to show that IQ do not measure something related to the brain size, I saw papers shared by an hereditarianssaying that “g” is highly related to brain size, I’ll try to find it again.

Maybe we could also tie that to the paper “what does IQ test test?”, but Emil pointed out the small sample size of some of Richardson’s references, it’s true that making hard conclusion based on some of his references would be dishonest but I don’t think that it should be ignored, especially when other articles/references like yours also point out problems.

Of the 104 individuals who had their heads scanned, volume for the cerebellum ranged from 735 cc in a 10 year old boy to 1470 cc in a 14 year old boy*

I think you mean the entire brain? The cerebellum is a small part of the brain

These cases deserve an explanation, for if large brains lead to high IQs, why do these people with heads significantly smaller have IQs in the normal range?*

Although some pinheads can have normal IQ, in the aggregate there is a positive correlation between head size and IQ, and most pinheads are below-average in IQ. I suppose it has also to do with the wiring of the brain…the density of neurons, the width of the axions and so on.

having a larger brain can also prevent Alzheimer’s and other age-related dementia

Although some pinheads can have normal IQ, in the aggregate there is a positive correlation between head size and IQ, and most pinheads are below-average in IQ. I suppose it has also to do with the wiring of the brain…the density of neurons, the width of the axions and so on.

I don’t disregard the positive .4 correlation between brain size and IQ. I cited the most recent study on the matter. I agree with you on density of neurons etc. That’s what matters most, not the overall size of the brain.

So if we can have brains the size of erectus and have normal IQ then brains must have expanded for another reason which Skoyles argues is expertise capacity. Information is stored in the cortical columns of the brain and larger brains have more cortical columns. Big brains also cause big problems (see Skoyles and Sagan, 2002: 240-244). Among other things, one of the biggest costs is the mother potentially dying giving birth. So due to these costs, there is a reason we have big brains.

I find your repeated casual use of the word ” pin head ” to describe someone like me somewhat offensive and distasteful. I am of normal intelligence , have a good job and live a perfectly normal life and reading your words makes my blood boil. You don’t have much grey enlightenment if you think that only large brained people like you will be reading this . I’m not saying everyone has to be ultra politically correct but surely there is no need to use such a horrible and condescending way to describe ” Micro cephalic ” people.

“Over thirty years ago, the National Institutes of Health funded a monumental study, the US National Collaborative Perinatal Project, which enrolled more than 54,000 pregnant women at 12 cooperating institutions across the country. Prenatal, perinatal and neonatal information was collected on the women and their infants, and the children were subsequently followed for up to seven years. Each child was examined at birth, four months, one year, four years and seven years, with physical measurements being taken at each of these ages. Full batteries of developmental tests, including the Wechsler Intelligence Scale, were administered at regular intervals. Using a sample of 35,704 children drawn from this study, Camp et al. (15) identified that microcephaly at birth was associated with a 2.35-fold increase in mental retardation. However, this risk was only observed in African American infants in families with low socioeconomic status.

Persistent microcephaly was a much more ominous finding. Using data from 28,820 term infants with at least three head circumference measurements between birth and seven years and excluding infants with neural tube defects, hydranencephaly, craniosynostosis or Down syndrome, Dolk (16) studied the outcome of infants who had OFCs greater than 2 standard deviations below the mean. The children who fared worst were those with a head circumference consistently more than 3 standard deviations below the mean. Of these children, 51% were mentally retarded (IQ =< 70) and 17% had an IQ of 71-80. Within this group of children, of those with some further specification of pathology such as a syndrome, congenital infection, etc., 94% were mentally retarded. Among those with microcephaly as the only diagnosis, only 25% were mentally retarded. Head circumference consistently between 2 to 3 standard deviations below the mean was associated with retardation in 11% of cases and IQ of 71-80 in 28%. Only 2.6% of children with head circumferences within the normal range were mentally retarded and 7.4% had IQs of 71-80. Another study, relying on head growth of 9,379 children enrolled in the study, looked at the ability of head size at one year to predict IQ at four years (17). At four years, IQ varied directly with head circumference. Head size at one year, at less than 43 (-2.3 SD) for males and 42 (-2.4 SD) for females, was associated with an IQ < 80 in approximately half the cases. None of these children achieved an IQ of 120 or greater at four years of age (17).”

No I don’t. That wasn’t even the point. I’m sure you get the point. I won’t insult you by repeating it again. Obviously I don’t think every case of microcephaly means normal IQ, it’s just the existence of these outliers deserves explanation, and I think the explanation is that most of our brain mass post-erectus isn’t needed for our high cognition and so must have increased for another reason, be it expertise capacity (a large brain would be needed for more cortical columns in which information is stored), protection against head trauma/TBI, hemispherectomies, etc.

The existence of people who are able to have average/above average IQ and half their brains needs an explanation. If the whole brain isn’t needed for a normal IQ and erectus brains are ‘good enough’, then I believe we must look at another explanation for human brain size increase. Erectus developed fire about 1.5 mya which is about the time when our brains really began increasing. This is due to endurance running, new pressures, etc and imo selection for expertise capacity and protection against TBI amongst other smaller, ancillary reasons.

Skoyles and Sagan’s arguments about Nariokotome boy are also sound.

So how well equipped was Homo erectus? To throw some figures at you (calculations shown in the notes), easily well enough. Of Nariokotome boy’s 673 cc of cortex, 164 cc would have been prefrontal cortex, roughly the same as half-brained people. Nariokotome boy did not need the mental competence required by cotemporary hunter-gatherers. … Compared to that of our distant ancestors, Upper Paleolithic technology is high tech. And the organizational skills used in hunts greatly improved 400,000 years ago to 20,000 years ago. These skills, in terms of our species, are recent, occurring by some estimates in less than the last 1 percent of our 2.5 million year existence as people. Before then, hunting skills would have required less brain power, as they were less mentally demanding. If you do not make detailed forward plans, then you do not need as much mental planning abilities as those who do. This suggests that the brains of Homo erectus did not arise for reasons of survival. For what they did, they could have gotten away with much smaller, Daniel Lyon-sized brains.

“Not needing all that brain mass for IQ and normal functioning, but specifically needing the PFC for long term memory, etc.”

Depends on what you mean by normal functioning. If you live an automechanica existence like most animals then sure, it isn’t needed. Of course brain mass is needed for IQ. Increased plasticity will allow an individual to develop more efficient neural connections.

Are you seriously suggesting our added brain mass is specifically for memory? (no need to categorize since short term and working memory and aren’t actually things.) That seems oddly specific and misses the bigger overall function that is redundancy.

This may sound confusing but the brain is both an integrated and a parallel system.

No because larger brains contain more cortical columns and cortical columns are where information is stored so people with larger brains should, theoretically, have a higher capacity for expertise due to having more cortical columns in which to store informational chunks.

What I’m saying is expertise capacity couldnt of increased because of “survival” reasons if brain size didn’t. Survival is a poor term anyway, you have to survive in a social group right? Our extra brain mass has intellectual benefits.

Please answer my question, what is the difference between g and expertise capacity?

What I’m saying is expertise capacity couldnt of increased because of “survival” reasons if brain size didn’t

In the context of the quote from Skoyles and Sagan, for what nation Nariokotome boy had to do, he didn’t need a brain much larger than Daniel Lyon because “hunting skills would have required less brain power, as they were less mentally demanding“.

Please answer my question, what is the difference between g and expertise capacity?

Capacity for expertise isn’t captured by IQ tests. Information is stored on cortical columns in the PFC, and larger brains have more cortical columns in which to stored information. So if IQ tests don’t test capacity for expertise, and informational chunks from becoming an expert are stored after long hours of doing a task while needing larger brains for more cortical columns, then you can see an at alternate reasons for why our brains increased (we agree on other causes for the increase but not all of it has to be for reason of ‘cognition’).

I’m aware. From what I understand working memory is more of a concept than a biological entity like short term memory which is itself a product of a false dichotomy of the same system, namely the cerebral cortex. Working memory involves the manipulation of retrieved sensory data.

[…] Ankney, 2007; Rushton and Ankney, 2009) for cognitive ability, though I have compiled numerous data that shows that we can have smaller brains and have IQs in the normal range, implying that large brains are […]